1. Field of the Invention
The present invention relates to a method for reproducing a halftone by controlling the number of dots in one picture element or a so-called density modulation system, and more particularly to a method for reproducing a halftone image by using a plurality of dots of different densities. The present invention also relates to a method for reproducing a halftone image capable of reproducing a color image.
2. Description of the Prior Art
In prior-art, density modulation methods for reproducing a halftone image by controlling the number of dots in one picture element, such as a dither method or a density pattern method, dots having the same density are used.
For example, one picture cell comprises 3.times.3=9 cells and each cell can contain one dot. Accordingly, ten tone levels (including non-dot pattern) are possible. However, a high quality of image cannot be reproduced with ten tone levels. Thus, the number of cells may be increased but this results in the increase of the size of picture cell and a reduction in resolution.
It has been proposed to use a systematic dither method without increasing the number of cells to reproduce multiple tone levels. However, in such a systematic dither method, a pseudo-pattern may be generated or image density may be coarsened at a highlight area and hence a high quality image cannot be obtained.
In order to resolve the above problems, it has been proposed to increase the number of densities of the colorant to obtain a wide range of tonality. As the number of densities increases, the reproduction range of the tonality is widened and the pseudo contour due to a difference between the densities can be prevented.
However, the number of heads increases accordingly, the signal processing circuitry becomes complex and a cost of the device increases. Further, the increase in the number of heads leads to the reduction of a reliability.
Thus, while the use of as many colorants as possible having different densities for one color is desirable to improve the image quality, the use of as few colorants as possible is desirable from the standpoints of cost and reliability.